If the reward always happens in front of the handler, that's where the dog will be. If the reward always happens 40 yards away, that's where the dog will be. If the reward always happens on the left side, that's where the dog will be.
Dogs gravitate to where the reward happens.
Bob Bailey said something to the effect of Mark for Behavior and Reward for Position (hat tip to Julie Jenkins). It's a very effective training tool if the trainer takes advantage of it. It's also a tremendous liability if one doesn't understand it. Not many people playing disc with their dogs understand this concept.